The present invention relates to a method for generating reliability information for a channel decoder of a radio receiver, particularly a mobile radio receiver, and to a corresponding radio receiver.
Transmission channels in mobile radio systems are characterized by their time-dependent multipath reception, which leads to intersymbol interference in the case of digital transmission systems. In order to be able to control such intersymbol interference, equalization of the received data is required at the receiving end. At the transmitting end, the data to be transmitted are transmitted interleaved and channel-coded due to the rapidly changing transmission conditions and for suppressing adjacent-channel and co-channel interference.
For the channel decoding at the receiving end, it is desirable to have information that specifies the reliability of the equalization performed by the equalizer. This reliability information is information that is obtained by a so-called soft decision. In contrast to a hard decision in which only a fixed decision threshold is used, a multiplicity of decision thresholds is used in the case of a soft decision, which distinctly increases the reliability of the decision. Equalizers as used, for example, in GSM receivers and also provided in accordance with the future expansion of the GSM mobile radio standard, EDGE, therefore must adequately equalize the received signal on the one hand, and, on the other hand, provide the aforementioned reliability information.
Many different methods for generating the aforementioned reliability information are known and in mobile radio systems, algorithms are frequently used which are based on a so-called Maximum Likelihood Sequence Estimation (MLSE) described, for example, in xe2x80x9cDigital Communicationsxe2x80x9d, Proakis, J. G., McGraw-Hill, New York, 1983. The most widely used implementation of this method is the Viterbi algorithm with which the aforementioned reliability information is obtained with the aid of trellis diagrams, in the form of probabilities of whether a received symbol is based on a transmitted xe2x80x980xe2x80x99 or a transmitted xe2x80x981xe2x80x99.
However, since this (optimum) algorithm is very complex and, as a result, very computationally intensive, and requires very large storage space, various sub-optimal methods have been developed that provide reliability information for the channel decoder in a simpler manner.
Such a sub-optimal method is described, among other things, in xe2x80x9cOptimum and Sub-optimum Detection of Coded Data Disturbed by Time-Varying Intersymbol Interferencexe2x80x9d, Wolfgang Koch and Alfred Baier, 1990 IEEE. According to this method called xe2x80x9cReduced State Soft Decision Equalizerxe2x80x9d, the reliability information is generated symbol by symbol in the equalizer. The corresponding algorithm is very similar to a hard decision Viterbi algorithm but it generates the reliability information in a distinctly simpler manner, the reliability information for a received symbol at time xcexcxe2x88x92L being determined at a time xcexc. L here designates the length of the observation period and corresponds to at least the length of the channel impulse response of the transmission channel. The reliability information is determined by determining the best xe2x80x9cone pathxe2x80x9d of the trellis diagram (i.e., the best or most advantageous path having the value xe2x80x981xe2x80x99 at time xcexcxe2x88x92L) and the best xe2x80x9czero pathxe2x80x9d (i.e., the best or most advantageous path having the value xe2x80x981xe2x80x99 at time xcexcxe2x88x92L) by means of a trellis diagram. These two paths of the trellis diagram are determined by means of metrics that are calculated for the individual state transitions in the trellis diagram. In this method, in particular, the so-called xe2x80x9cmatched-filterxe2x80x9d metric is used. Finally, the reliability information is obtained by putting the metrics calculated for the best xe2x80x9cone pathxe2x80x9d and best xe2x80x9czero pathxe2x80x9d in this manner in relation to one another. To reduce the computational expenditure and the storage requirement, a trellis having a reduced number of states is used for calculating the individual metrics. A trellis-based equalization is only started for elements 0 . . . Lxe2x80x2 (Lxe2x80x2 less than L) of the channel impulse response whereas the remaining elements Lxe2x80x2+1 . . . L are only included in the trellis-based equalization in a decision-feedback manner. The principles of this decision feedback (Decision Feedback Sequence Estimation) can be found, for example, in the paper xe2x80x9cReduced-state Sequence Estimation with Set Partitioning and Decision Feedbackxe2x80x9d, Vedat Eyuboglu and Shahid Qureshi, 1988 IEEE.
In the procedure described above, the equalizer must determine the best xe2x80x9cone-pathxe2x80x9d and xe2x80x9czero pathxe2x80x9d with reference to time xcexcxe2x88x92Lxe2x80x2 at each time xcexc and calculate from these determinations the reliability information for the received symbol at time xcexcxe2x88x92Lxe2x80x2. In this process, the branch metrics include bits at times xcexc . . . xcexcxe2x88x92Lxe2x80x2 and bits at times xcexcxe2x88x92Lxe2x80x2xe2x88x921 . . . xcexcxe2x88x92L, the latter bits, as already described, being included in the metrics calculation as decision feedback. These latter bits are obtained from the individual so-called xe2x80x9csurvivorxe2x80x9d paths of the 2Lxe2x80x2 states of the trellis diagram (i.e., the most inexpensive and most probable state transitions in each case), which, however, are different from state to state as a consequence, so that a correspondingly high computational effort and storage requirement is needed since the equalizer must carry a list with 2Lxe2x80x2 states at each time xcexc.
The presently disclosed method and apparatus provide generation of information for channel decoding in a radio receiver wherein the computational expenditure and the storage space needed for computations are reduced.
Specifically, a method for generating reliability information for channel decoding in a radio receiver is disclosed wherein reliability information (q) specifies probabilities of a data symbol (z) received by the radio receiver via a radio channel based on one of first and second values that are transmitted. The method includes determining reliability information (q) for a time xcexcxe2x88x92Lxe2x80x2 at an arbitrary time xcexcfor each received data symbol (z) by determining through the use of a state model having 2Lxe2x80x2 states a first path that most probably contains the first value at time xcexcxe2x88x92Lxe2x80x2. Also a second path which most probably contains a second value at time xcexcxe2x88x92Lxe2x80x2 is determined and metrics calculated for the first path and the second path are placed into a relationship with one another wherein metrics calculated for the first path and the second path are calculated in dependence on a first group of symbols of the state model present at time xcexc . . . xcexcxe2x88x92Lxe2x80x2 and a second group of symbols of the state model present at times xcexcxe2x88x92Lxe2x80x2xe2x88x921 . . . xcexcxe2x88x92L and L corresponding to at least the length of a channel impulse response of the radio channel with L greater than Lxe2x80x2. Furthermore, the method includes the step of utilizing a value that has been decided before the time xcexcxe2x88x92Lxe2x80x2 and is identical for all states of the state model as symbols of the second group for determining the reliability information (q) for time xcexcxe2x88x92Lxe2x80x2.
An apparatus for use in a radio receiver is also disclosed that includes an equalizer configured to equalize a radio signal received via a radio channel and generate reliability information (q) for a downstream channel decoder, wherein the reliability information (q) specifies probabilities of a received data symbol (z) based on at least one of a first and second transmitted value. The equalizer also determines for each received data symbol (z) at an arbitrary time xcexc a reliability information item (q) for a time xcexcxe2x88x92Lxe2x80x2 by determining, by means of a state model with 2Lxe2x80x2 states, a first path that most probably contains the first value at time xcexcxe2x88x92Lxe2x80x2 and a second path that most probably contains the second value at time xcexcxe2x88x92Lxe2x80x2. The equalizer further places metrics calculated for the first path and the second path in relationship with one another. Additionally, the equalizer calculates the metrics calculated for the first path and the second path in dependence on a first group of symbols of the state model present at times xcexc . . . xcexcxe2x88x92Lxe2x80x2 and a second group of symbols of a state model present at times xcexcxe2x88x92Lxe2x80x2xe2x88x921 . . . xcexcxe2x88x92L, with L corresponding to at least the length of the channel impulse response and the radio channel with L greater than Lxe2x80x2. Finally, the equalizer is also configured to utilize the value of the reliability information that was decided before time xcexcxe2x88x92Lxe2x80x2 and is identical for all states of the state model as symbols of the second group in order to determine the reliability information (q) for the time xcexcxe2x88x92Lxe2x80x2 symbols.